B737-300 CREW WAS UNABLE TO RETRACT THE LNDG GEAR BECAUSE OF THE GEAR LEVER SAFETY LEVER LATCH WAS NOT RETRACTED.

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AN ACFT MADE AN AIR-RETURN BECAUSE OF 'LNDG GEAR WOULD NOT RETRACT ON TKOF. LEVER WOULD NOT SELECT BEYOND THE 'OFF' POS.' I TROUBLESHOT THE PROB WITH ANOTHER MECH. WE DETERMINED THAT THE CAUSE OF THE PROB WAS; S106; THE LNDG GEAR SAFETY SWITCH (AIR SENSING PROX SWITCH). I REMOVED THE PROX SWITCH AND SPLICED IN ANOTHER NEW SWITCH. WE ALSO REPLACED THE M991; AIR SENSING SQUAT SWITCH CARD AND R276 AIR SENSING RELAY. ANOTHER MECH PHYSICALLY INSTALLED THE SWITCH AND CHKED THE CLRNC (RIG) BTWN THE TARGET AND THE SWITCH. I MOVED THE TELEFLEX CABLE THAT CHANGED THE POS OF THE TARGET FROM GND TO AIR. I LACED AND TIE-WRAPPED THE WIRING BUNDLE TO THE PROX SWITCH AND INSTALLED THE PROTECTIVE COVER. I DID PERFORM AN ELECTRICAL CHK OF THE S106 PROX SWITCH FROM THE EII RACK IN THE FORWARD E&E COMPARTMENT WITH SWITCH S1. I WAS GOOD. I SHORT SIGNED THE ACFT LOGBOOK STATING WHAT WAS DONE AND THAT THE SWITCH OPERATIONALLY CHKED OK. I HAD INADVERTENTLY SIGNED FOR ANOTHER MECH'S WORK -- THE INSTALLATION; CHK-OUT; AND RIG OF SWITCH S106. I FOUND OUT AFTER THE 3RD AIR-RETURN; THAT THE MECH WHO INSTALLED THE SWITCH; DID NOT KNOW WHICH SIDE OF THE PROX SWITCH WAS THE ACTUATING SURFACE. I SHOULD HAVE RECHKED HIS WORK BEFORE SIGNING THE LOGBOOK. I ASSUMED THAT HE WAS AN EXPERIENCED ACFT MECH; AND THAT I DID NOT HAVE TO CHK HIS WORK. HE DID NOT EXPRESS ANY DOUBTS ABOUT THE INSTALLATION. IT WAS MY ERROR IN JUDGEMENT TO NOT TAKE THE TIME TO XCHK ANOTHER MECH'S WORK BEFORE SIGNING THE ACFT'S LOGBOOK. SUPPLEMENTAL INFO FROM ACN 520714: ACFT AIR RETURN TO BASE FOR GEAR WOULD NOT RETRACT. REPLACED NEUTRAL SHIFT SENSOR; PRESUMED ELECTRICAL CHECK WAS OUT. DURING ELECTRICAL CHECK OUT MECH IN THE FLT DECK RAISED THE GEAR WITH GEAR OVERRIDE TRIGGER. THIS IS NOT REQUIRED WHEN RAISING THE GEAR WHEN THE PLANE IS ON JACKS. THIS FAILED TO SHOW THE SENSOR THAT WAS INSTALLED WAS NOT PROPERLY INSTALLED.